1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breaker trip units.
2. Background Information
Circuit breakers and circuit breaker trip units are well known in the art. Resetting of a circuit breaker (e.g., through the operating handle and operating mechanism thereof) is also accomplished in a manner well known in the art. Generally, a circuit breaker includes an operating mechanism structured to move a number of separable contacts between an open, first position and a closed, second position. The operating mechanism may be actuated manually or by the trip unit.
The trip unit includes an over-current sensor, a trip actuator, and a trip bar. Generally, the over-current sensor is structured to detect an over-current condition in the conductors of the circuit breaker. The over-current sensor may be a mechanical device, an electrical device, or a combination thereof. In an exemplary embodiment, the over-current sensor produces an electronic signal upon detecting an over-current condition. The trip actuator is an electro-mechanical apparatus that operates various parts of the trip unit after being activated by the trip signal. That is, the trip actuator receives the signal from the over-current sensor and produces a mechanical motion. In an exemplary embodiment, the trip actuator includes an elongated plunger that moves longitudinally. The trip actuator acts upon the trip bar.
The trip bar is an elongated generally cylindrical member structured to rotate about an axis of rotation. The trip bar includes a number of extensions, e.g. radial extensions and tangential extensions, that interact with other components of the trip unit and circuit breaker. For example, the trip bar is coupled to the circuit breaker operating mechanism and, when actuated by the trip actuator, rotation of the trip bar causes the operating mechanism to move the contacts from the second, closed position to the first, open position. That is, the trip bar is part of the linkage that allows the trip unit to trip the circuit breaker.
A trip unit also includes a housing assembly that substantially encloses the other trip unit components. The circuit breaker housing assembly includes a cavity into which the trip unit is disposed. That is, the circuit breaker housing assembly cavity is sized to correspond to the trip unit housing assembly. The trip unit housing assembly is divided into two halves, each half including a planar member with a peripheral, generally perpendicular depending sidewall. Thus, when the two halves are brought together, the housing assembly defines an enclosed space for the other components.
This design has disadvantages in that alignment and tolerance error allowed the trip bar to be pinched or misaligned. That is, for example, as shown in U.S. Pat. No. 6,853,279, the trip bar is disposed in a saddle extending from one housing assembly sidewall. A trip detection circuit is disposed over the trip bar. Then the other housing assembly sidewall is coupled to the first housing assembly sidewall, sandwiching the components there between. In this configuration, the trip unit housing assembly must define multiple spaces for the internal components, each of which have tolerances built into the separate housing assembly sidewalls. The requirement of multiple tolerances can allow the trip bar to have too little or too much space. Too much tolerance allows the trip bar to be loose and allows for “rattle.” If the trip bar is loose, there may be either too much “latch bite,” which requires more force to trip, or, too little “latch bite,” which promotes premature tripping. Further, too many tolerances (tolerance build up/stack up) also result in inconsistent latch loading and yield issues for accessories.